Re: document rotation

Dominik wrote:

> i want to detect rotation angle of scanned document (simple text no
> graphics)
> anyone knows preprocessing algorithms that deal with that?

  Just some random ideas:

  Input is bitmap.

  1. Segment the image.  Result is an 'unordered' set of bitmaps.
     Depending on the alphabet, you may be able to deduce the general direction
     of the lines: western letters are typically high and narrow. Segments
     that are broader than expected may be joined letters.

  2. Group the segments in several passes: first pass is nearest neighbor,
     this is assumed to be equivalent of words.  From each word you get
     one or more hypotheses about baseline position and direction.
     Second pass is nearest neighbour among the groups from first pass, keeping
     best guess of baselines aligned: this gives you lines or parts of lines.

   3. Figure out the baseline direction of the lines.

  Or perhaps: Do some kind of averaging operation. This gives you grey bands
instead of words. Threshold, skeletize and calculate directions.
(Any too large segments after thresholding are probably pictures or
lines). Or perhaps edge-detect instead of thresholding (in grey-scale, not binary).

--
Anders Thulin     [EMAIL PROTECTED]     http://www.algonet.se/~ath